Apparatus for regulating the depth of penetration of a plough

ABSTRACT

An apparatus for regulating the depth of penetration of a plow which is drawn by a tractor and lifted or lowered by a hydraulic piston-cylinder arrangement. The depth of penetration of the plow is regulated by controlling the pressure in the working chamber of the lifting cylinder.

United States Patent [191 1 3,731,744 Lipinski [4 1 May 8, 1973 [54]APPARATUS FOR REGULATING THE [56] References Cited DEPTH OF PENETRATIONOF A PLOUGH UNITED STATES PATENTS 75 r J h Lipinski, 1030 Vienna3,285,140 11/1966 Annat et al. ..91/433 X 1 3,004,610 10/1961 Pomper etal. ..172/7 [73] Assignee: Alex Friedmann, Vienna, Austria PrimaryEmmmerRobert pulfrey [22] Filed: Dec. 21, 1971 Assistant Examiner-Stephen C.Pellegrino pp No: 210,361 Attorney-Marvin R. Stern [57] ABSTRACT [30]Foreign Apphcatmn Priority Data An apparatus for regulating the depth ofpenetration Dec. 23, 1970 Austria ..A 11624/70 of a plow which is drawnby a tractor and lifted or lowered by a hydraulic piston-cylinderarrangement. U-S. Cl. The of penetration of the plow is regulated [51]Int. Cl. ..Aolb 63/112 controlling the pressure in the working chamber fthe [58] Fleld of Search ..172/9, 7, 8; 91/390, lifting cy1inder 6Claims, 13 Drawing Figures Patented May 8, 1973 3,731,744

3 sheets sheei 1 Patented May 8, 1973 3,731,744

3 Sheets-Sheet 2 Patented May 8, 1973 3,731,744

3 Sheets-Sheet 5 APPARATUS FOR REGULATING THE DEPTH F PENETRATION OF APLOUGH This invention relates to an apparatus for regulating the depthof penetration of a plough which is drawn by a tractor and which may belifted or lowered by a hydraulic piston-cylinder arrangement. Inconnection with such ploughs the level of which can be varied by meansof a piston-cylinder arrangement of the tractor normally it is necessaryto lower the plough gradually until the desired depth of penetration isreached; this procedure, however, requires some experience. Furthermore,methods and apparatus are known whereat the depth of penetration isbeing reduced as a function of an increasing reacting force put up tothe plough moving through the soil to avoid an overload of the tractorand drawbar. Such apparatus for instance may operate in dependence ofpower or traction, and in case of exceeding a predetermined power ortraction value the plough is being lifted.

It has also been suggested to regulate the depth of penetration of aplough in dependence of the pressure in the working chamber of thelifting cylinder. The plough is being held at its level by the piston insaid lifting cylinder, and the reacting forces put up to the plough inthe soil cause a variation of the lifting force for holding the ploughwhich results in a variation of the pressure within the liftingcylinder. An increase of the aforesaid reacting forces may cause adecrease of the lifting force and of the pressure in the working chamberof the lifting cylinder, or an increase of these values, which dependsupon the construction of the plough. In case of a plough of conventionalconstruction, the said reacting forces cause an uplift of the plough,and an increase of the reacting forces will reduce the lifting forcenecessary for holding the plough, and the pressure in the workingchamber of the lifting cylinder. A primary disadvantage encountered inthe heretofore known methods and systems for controlling the depth ofpenetration of a plough in dependence of the reacting forces put up tothe plough in the soil is that the lifting and lowering movement of theplough is uncontrollably great which often may lead to an unwantedcomplete withdrawal of the plough from the soil.

It is, therefore, a primary object of the invention to avoid theaforesaid disadvantages and to provide a novel apparatus for regulatingthe depth of penetration of a plough which is drawn by a tractor andwhich may be lifted or lowered by a hydraulic piston-cylinderarrangement, comprising a control piston controlling the supply ofhydraulic fluid to the lifting cylinder, said control piston beingloaded against a force by a hydraulic control pressure setting up in oneof its associated working chambers (control pressure chamber),adjustment of the control piston resulting from variation of aconstriction in a connection of the control pressure chamber with adrain in dependence of the pressure in the working chamber of thelifting cylinder, a medium throttling position of said constrictioncausing an intermediate position of said control piston in which theworking chamber of the lifting cylinder is closed, control means beingconnected in series before said constriction and consisting of a firstslide which may be moved at will, and a second slide cooperating withsaid first slide and being moveable in dependence of the lifting levelof the plough, the control ports of said slides in one relative positionthereof connect the control pressure chamber with a drain, and inanother relative posi-' tion thereof block the outlet of the controlpressure chamber whereat the distance between said two relativepositions determines the lifting range of the plough within which thelifting level of the plough can vary in dependence of the pressure inthe working chamber of the lifting cylinder, and the constriction meanswhich controls the outlet in dependence of the pressure in the workingchamber of the lifting cylinder is on one hand influenced by thepressure in the working chamber of the lifting cylinder and on the otherhand by an adjustable force counteracting to the force exerted by saidpressure, such as a spring having an adjustable support.

Thereby the range within which the depth of penetration of the plough inthe soil can vary in dependence of reacting forces put up to the ploughis being limited. Within this limited range the depth of penetrationvaries automatically in such a way that traction is being kept aboutequal, it is avoided, however, that in case of extreme decrease of saidreacting forces the plough will penetrate into the soil too deeply in anundesired manner, and that in case of excessive increase of saidreacting forces the plough will be completely withdrawn from the soilwhich would entail a portion of the soil being left unplowed. Withinsaid limited penetration-depth range the tractor operates with constanttraction, and after reaching the limits of said range a change intraction is accepted to avoid an undue discontinuity of soil treatment.

Preferably the two slides which are moveable relatively to each otherare flat rotary slides, the control ports of said slide cooperate withcontrol ports of said second slide, said control ports of said secondslide communicating with the control pressure chamber and cooperatingwith control ports in the slide face, one of said latter control portscommunicating with said constriction means being adjustable independence of the pressure in the working chamber of the liftingcylinder, and the other one communicating uncontrolledly with a drainchamber.

If now by a decrease of the throttling effect the con trol pressure inthe control pressure chamber causes movement of the plough beyond thepredetermined limiting position, opposite motion of the control pistonis being initiated by blocking the outlet of the control pressurechamber. The same applies to the case where upon increase of thethrottling effect in the outlet of the control pressure chamber theother predetermined limiting position is being reached, in which caseagain an opposite motion of the control piston is being initiated byopening the outlet of the control pressure chamber to the drain chamber.Thereby a balanced condition is achieved in the limiting positions untilthe plough is brought back to an intermediate position within thelimited penetration-depth range. The level of said range can be changedby adjusting said first slide which may be moved at will, whereupon saidsecond slide which is moveable in dependence of the lifting level of theplough, and thereby also the plough itself, will follow up the positionchosen by said first slide. To this end said second slide has onecontrol port which in one limiting position of the penetration-depthrange becomes aligned with a control port of said first slide whichcommunicates with the constriction means being adjustable in dependenceof the pressure in the working chamber of the lifting cylinder, andanother control port communicating with the control pressure chamber andin the other limiting position of the penetrationdepth range becomingaligned with a control port of said first slide which communicates witha control port in the slide face connected to the drain chamber, whereatthe angular displacement of the control ports of said second slide withrespect to the control ports of said first slide determines thepenetration-depth range of the plough.

The penetration-depth range shall be chosen so that the mechanicalresistance of .the soil put up to the plough can essentially beequalized, that, however, excessive lifting and lowering motion of theplough is avoided. Therefore the aforesaid angular displacement shallcorrespond to a penetration-depth range of the plough which is less thanthe maximum penetration depth, preferably about half the penetrationdepth of the plough.

The control ports of said first slide may be formed as slots which inall of said slides positions for controlling the level of the plough independence of the pressure in the working chamber of the liftingcylinder communicate with the constriction means adjusted dependence ofthe pressure in the working chamber and with the control port in theslide face connected to the drain chamber, respectively.

In a preferred embodiment of the apparatus according to the inventionsaid second slide is provided with a control recess communicating withthe control pressure chamber, and said first slide has a control slotwhich upon rotation of said first slide to a range of positions, wherethe connection between the control pressure chamber and the constrictionmeans adjusted in dependence of the pressure in the working chamber ofthe lifting cylinder is cut off, communicates with the control port inthe slide face connected to the drain chamber. Thereby it becomespossible to obtain either automatic control of the penetration depth ofthe plough in dependence of the mechanical resistance of the soil put upto the plough, within a range between predetermined levels ofpenetration, or a manual control of the depth of penetration of theplough, whatever is desired, by means ofa single band lever and only oneset of rotary slides.

Features and advantages of the invention will become apparent from thefollowing description of an embodiment thereof given, by way of example,in conjunction with the accompanying drawings in which:

FIG. 1 is a partly sectioned side elevation of an apparatus constructedin accordance with the present invention showing the control piston forcontrolling supply and drain of hydraulic fluid to and from the workingchamber of the lifting cylinder.

FIG. 2 is a schematic diagram showing the connections between thelifting cylinder with the piston for holding the plough and theapparatus of the present invention.

FIGS. 3a, 3b and 3c are isometric views of the two rotary slides and theslide face, resp., when taken apart.

FIG. 4 is a fragmentary sectional view of the slide face with assembledrotary slides.

FIGS. 5a, 5b and 5c are sideelevational views of corresponding positionsof the two rotary slides and the slide face for manual operation and theuppermost position of the plough.

FIGS. 6a, 6b and 6c are side elevation views of corresponding positionsof the two rotary slides and the slide face for manual operation and thelowermost position of the plough.

FIG. 7 is a schematic partly sectioned side elevation showing thearrangement of the rotary slides with the slide face, with the firstrotary slide which may be ad justed at will in the position forautomatic operation.

Referring to FIG. 1 of the drawings, the apparatus is being suppliedcontinuously with hydraulic fluid through a supply line 1. In a housing2 there is slidably supported a control piston 3 on which a restoringforce is exerted by a coiled spring 4. The working chamber 5 of alifting cylinder 6 as shown in FIG. 2 is connected to an annular controlrecess 8 via a conduit 7. Said recess surrounds the control piston whichin the position shown in FIG. 1 closes the recess. A check valve 9 willopen only in the direction from supply line 1 to control recess 8 butwill stay closed in case of excess pressure in the working chamber 5 oflifting cylinder 6; then the working chamber of the lifting cylinder issealed off and lifting piston 10 is blocked in its position. Hydraulicfluid from the supply line 1 also flows into a second control recess 11which in the neutral position of control piston 3 via a radial bore 12thereof communicates with an axial bore 13 within said control piston.The left end of said axial bore opens out into a chamber 14, as shown inFIG. 1, which chamber is connected to a return line 16 via a pressureholding valve 15. With this return line 16 there is also connected athird annular control recess 17 surrounding the control piston 3. In theoutermost right position of control piston 3 which causes lowering ofthe plough recess 17 communicates with recess 8 whereby hydraulic fluidcan flow out of the working chamber 5 of lifting cylinder 6. In the leftposition of control piston 3 which causes lifting of the plough theconnection between radial bore 12 and control recess 11 is cut off andthe hydraulic fluid supplied through supply line 1 can reach the axialbore 13 of control piston 3 only through a throttle 18. For this reasonmore hydraulic fluid is supplied via check valve 9 and conduit 7 intoworking chamber 5 of lifting cylinder 6.

From axial bore 13 of control piston 3 hydraulic fluid flows throughanother throttle 19 into pressure control chamber 20 which is located atthe right end of the control piston as shown in FIG. 1 and whichcontains the coiled spring 4. In dependence of the outlet conditions ahydraulic pressure will build up in the pressure control chamber 20which pressure adds to the force of spring 4, and the resulting forcewill counter-balance the hydrostatic pressure from the other side of thecontrol piston in its neutral position.

In the drain conduit 26a of control pressure chamber 20 a constrictionmeans 27a is provided the throttling cross section of which iscontrollable by different control signals. If the throttling crosssection is being reduced the pressure in the pressure control chamber 20will rise whereby the control piston 3 is brought into position LIFT,and if the throttling cross section is being increased the pressure incontrol chamber 20 will drop whereby control piston 3 is brought intoposition LOWER. Next to the pressure control chamber 20 in its drainconduit there is an arrangement of rotary slides, and thereafter athrottling cross section is provided which is controllable in dependenceof the pressure in working chamber 5 oflifting cylinder 6.

FIG. 3 shows the slide arrangement when taken apart. Reference numeral21 designates a slide (second slide) which is adjusted in dependence ofthe lifting level of the plough by means of a feedback mechanism, andreference numeral 22 designates a slide (first slide) which can beadjusted at will by means ofa hand lever 23 to choose the penetrationdepth of the plough. The two slides are arranged coaxially in a recesswhich is provided in housing 2 and has a slide face 24. Slide face 24 isshown in FIG. and has two outlet ports. One port, 26, thereof isconnected directly with the drain, and the other one, 27, is connectedwith the constriction which is controlled by the pressure in the workingchamber 5 of lifting cylinder 6. The first slide 22 shown in FIG. 3b hason its face opposite slide face 24 in recess 25 three arcuate controlsectors. One of said sectors is a slot 28 passing through the slide,while the other two sectors are formed by grooves facing the slide face24 whereat groove 29 at one of its ends through a port 31 opens to theother face of the slide, and groove 30 opens to the other face of theslide through a port 32 offset radially inwardly with respect to thegroove. On said other face of slide 22 rests the second slide 21 shownin FIG. 3a which has a slotted port 33 in the form of a sector, and tworadial control slots 34 and 35 which are open to the circumference, too.The second slide 21 is being rotated clockwise if the plough is lowered,and counterclockwise if the plough is lifted. FIG. 4 shown an axialsection through the slides and the surrounding portion of housing 2 fromwhich it can be seen that the control pressure chamber 20 lying at oneend of control piston 3 communicates with the recess 25 which surroundsthe circumferences of both slides and the front face of the second slide21. Therefore slotted port 33 and radial control slots 34 and 35 ofslide 21 always are filled with hydraulic fluid flowing off frompressure control chamber 20.

The apparatus of the present invention allows two different methods ofoperation for controlling the position of the plough. One method ofoperation is to exactly position the plough with only small toleranceupon a level preselected by the position of the hand lever 23. In FIG. 2the angle a is the angle of adjustment of the second slide 21corresponding to the total range of adjustment of the plough which isconnected with said slide by means of a feedback mechanism. For themanual control of the depth of penetration of the plough hand lever 23may be rotated within an angle [3. For the other method of operation,namely automatic control of the depth of penetration, hand lever 23 maybe rotated within an angle'y. When using that latter method ofoperation, second slide 21 will be caused to follow to the positionselected by hand lever 23, too, however with the provision that thisprimary control is superimposed by a control in dependence of themachanical resistance of the soil put up to the plough so that within apredetermined tolerance range the plough can be retracted by a certainamount in case of highly increasing mechanical resistance of the soil.

FIG. 5 shows the relative positions of the two slides with respect ofthe slide face for manual operation and the uppermost position of theplough. The usable angular range for manual positioning of slide 22 islimited by arc length of slot 28 which has to allow communication withdrain port 26 in slide face 24. Follow-up control of the plough isachieved by the fact that control piston 3 only will stay in its neutralposition if the drain of control pressure chamber 20 is being throttledin a predetermined manner. The necessary throttling cross section isobtained by the control slot 34 of slide 21 cooperating with the edge ofslot 28 in slide 22. As long as the position of the plough preselectedby hand lever 23 is not reached exactly, or after any readjustment ofhand lever 23, resp., the drain cross section of control pressurechamber 20 is passingly either fully open or completely closed wherebycontrol piston 3 is brought into position LOWER or into position LIFT,resp., for a period of time until second slide 21 moves its control slot34 over the end of slot 28 in slide 22 so that the drain of controlpressure chamber 20 is being throttled properly whereupon control piston3 will balance in its neutral position. Operation of the apparatus incase of the lowermost level of the plough with the relative positions ofthe two slides with respect to the slide face as shown in FIG. 6 isanalogous.

For an explanation of automatic operation, reference is made to FIG. 7.In this case control slot 34 of slide 21 and slot 28 of slide 22 areinoperative. Groove 30 of the first slide 22 communicates with drainport 27. Sectorshaped control port 33 of the second slide 21 cooperateswith port 32 which communicates with groove 30 of slide 22, and controlslot 35 of slide 21 cooperates with port 31 which communicates withgroove 29 of slide 22. In this case of the preselected level of theplough has a predetermined tolerance range within which the plough mayadjust itself in dependence of the mechanical resistance of the soil,and said tolerance range is determined by the sum of angles and ii:shown in FIG. 7. Port 27 in slide face 24 is not a direct drain butleads through a constriction means which is controlled in dependence ofthe pressure in the working chamber 5 of the lifting cylinder 6 andwhich comprises a small plunger 36 with a cooperating throttle element37 in the drain, said plunger 36 being loaded by the pressure in theworking chamber of the lifting cylinder. The throttle element 37 isloaded by a spring 38 on its end opposite the plunger 36. The other endof said spring rests on an adjustable support 39. By adjustment of saidsupport different plough weights can be compensated so that thetolerance range always will lie symmetrical with respect to the levelbeing set by means of hand lever 23. If due to the soil conditions thereacting forces put up to the plough will rise and tend to lift theplough, there will be a reduction of pressure in working chamber 5 oflifting cylinder 6. As a result of such reduced pressure plunger 36 andthrottle element 37 in the arrangement of FIG. 7 will be shifted to theleft, and the drain of hydraulic fluid from control pressure chamber 20of control piston 3 will be reduced. Thereby control piston 3 comespassingly into its position LIFT, however, for the most for such aperiod of time until one end of slotted port 33 in slide 21 moves overport 32 in slide 22 whereby control pressure chamber 20 is connecteddirectly with the drain.

In the contrary case, namely an increase of pressure in working chamberof lifting cylinder 6, for instance when passing a depression on thesurface of the soil, plunger 36 will open throttle element 37 againstthe restoring force of spring 38 so that the pressure in the controlpressure chamber will decrease, and control piston 3 passingly will comeinto its position LOWER. Lowering of the plough, however, only ispossible as long as control slot 35 of slide 21 is at least partlyaligned with port 31 of slide 22, because if the drain were cut offcompletely by means of control slot 35 and port 31 the pressure inpressure control chamber 20 would rise until control piston 3 againwould come into its position LIFT.

The apparatus of the present invention allows either a precise andstrict control of the level of the plough, or preselection of a meanlevel whereat in dependence of the mechanical resistance of the soil theplough may automatically perform small lifting and lowering motionswithin a predetermined tolerance range.

Although the invention has been described in terms of a certain specificembodiment, it will be understood that other arrangements may be devisedby those skilled in the art which likewise will fall within the scopeand spirit of the invention.

What I claim is:

1. An apparatus for regulating the depth of penetration of a ploughwhich is drawn by a tractor and which may be lifted or lowered by ahydraulic piston-cylinder arrangement, comprising a control pistoncontrolling the supply of hydraulic fluid to the lifting cylinder, saidcontrol piston being loaded against a force by a hydraulic controlpressure setting up in one of its control pressure chamber, adjustmentof the control piston resulting from variation of a constriction meansin a connection of the control pressure chamber with a drain independence of the pressure in the working chamber of the liftingcylinder, a medium throttling position of said constriction meanscausing an intermediate position of said control piston in which theworking chamber of the lifting cylinder is closed, control means beingconnected in series before said constriction means and consisting of afirst slide which may be moved at will, and a second slide cooperatingwith said first slide through control parts and being moveable independence of the lifting level of the plough, the control ports of saidslides in one relative position thereof connecting the control pressurechamber with a drain, and in another relative position thereof blockingthe outlet of the control pressure chamber whereat the distance betweensaid two relative positions determines the lifting range of the ploughwithin which the lifting level of the plough can vary in dependence ofthe pressure in the working chamber of the lifting cylinder, and theconstriction means which controls the outlet in dependence of thepressure in the working chamber of the lifting cylinder is on one handinfluenced by the pressure in the working chamber of the liftingcylinder and on the other hand by an adjustable force meanscounteracting to the force exerted by said pressure.

2. An apparatus as claimed in claim 1, wherein said two slides which aremoveable relatively to each other are flat rotary slides, the controlports of said first slide cooperating with control ports of said secondslide, said control ports of said second slide communicating with thecon rol pressure chamber and cooperating with control ports in the slideface, one of said latter control ports communicating with saidconstriction means being adjustable in dependence of the pressure in theworking chamber of the lifting cylinder, and the other one communicatingwith a drain chamber.

3. An apparatus as claimed in claim 1, wherein said second slide has onecontrol port which in one limiting position of the penetration-depthrange becomes aligned with a control port of said first slide whichcommunicates with the constriction means being adjustable in dependenceof the pressure in the working chamber of the lifting cylinder, andanother control port communicating with the control pressure chamber andin the other limiting position of the penetration-depth range becomingaligned with a control port in said first slide which communicates witha control port in the slide face connected to the drain chamber, whereatthe angular displacement of the control ports of said second slide withrespect to the control ports of said first slide determines thepenetration-depth range of the plough.

4. An apparatus as claimed in claim 3, wherein said angular displacementcorresponds to a penetrationdepth range of the plough which is less thanthe maximum penetration depth, preferably about half the penetrationdepth of the plough.

5. An apparatus as claimed in claim 1, wherein the control ports of saidfirst slide are formed as slots which in all of said slides positionsfor controlling the level of the plough in dependence of the pressure inthe working chamber of the lifting cylinder communicate with saidconstriction means adjusted in dependence of the pressure in the workingchamber and with the control port in the slide face connected to thedrain chamber, respectively.

6. An apparatus as claimed in claim 1, wherein said second slide isprovided with a control recess communicating with the control pressurechamber, and said first slide has a control slot which upon rotation ofsaid first slide to a range of positions, where the connection betweenthe control pressure chamber and the constriction means adjusted independence of the pressure in the working chamber of the liftingcylinder is cut off, communicates with the control port in the slideface connected to the drain chamber.

1. An apparatus for regulating the depth of penetration of a ploughwhich is drawn by a tractor and which may be lifted or lowered by ahydraulic piston-cylinder arrangement, comprising a control pistoncontrolling the supply of hydraulic fluid to the lifting cylinder, saidcontrol piston being loaded against a force by a hydraulic controlpressure setting up in one of its control pressure chamber, adjustmentof the control piston resulting from variation of a constriction meansin a connection of the control pressure chamber with a drain independence of the pressure in the working chamber of the liftingcylinder, a medium throttling position of said constriction meanscausing an intermediate position of said control piston in which theworking chamber of the lifting cylinder is closed, control means beingconnected in series before said constriction means and consisting of afirst slide which may be moved at will, and a second slide cooperatingwith said first slide through control parts and being moveable independence of the lifting level of the plough, the control ports of saidslides in one relative position thereof connecting the control pressurechamber with a drain, and in another relative position thereof blockingthe outlet of the control pressure chamber whereaT the distance betweensaid two relative positions determines the lifting range of the ploughwithin which the lifting level of the plough can vary in dependence ofthe pressure in the working chamber of the lifting cylinder, and theconstriction means which controls the outlet in dependence of thepressure in the working chamber of the lifting cylinder is on one handinfluenced by the pressure in the working chamber of the liftingcylinder and on the other hand by an adjustable force meanscounteracting to the force exerted by said pressure.
 2. An apparatus asclaimed in claim 1, wherein said two slides which are moveablerelatively to each other are flat rotary slides, the control ports ofsaid first slide cooperating with control ports of said second slide,said control ports of said second slide communicating with the controlpressure chamber and cooperating with control ports in the slide face,one of said latter control ports communicating with said constrictionmeans being adjustable in dependence of the pressure in the workingchamber of the lifting cylinder, and the other one communicating with adrain chamber.
 3. An apparatus as claimed in claim 1, wherein saidsecond slide has one control port which in one limiting position of thepenetration-depth range becomes aligned with a control port of saidfirst slide which communicates with the constriction means beingadjustable in dependence of the pressure in the working chamber of thelifting cylinder, and another control port communicating with thecontrol pressure chamber and in the other limiting position of thepenetration-depth range becoming aligned with a control port in saidfirst slide which communicates with a control port in the slide faceconnected to the drain chamber, whereat the angular displacement of thecontrol ports of said second slide with respect to the control ports ofsaid first slide determines the penetration-depth range of the plough.4. An apparatus as claimed in claim 3, wherein said angular displacementcorresponds to a penetration-depth range of the plough which is lessthan the maximum penetration depth, preferably about half thepenetration depth of the plough.
 5. An apparatus as claimed in claim 1,wherein the control ports of said first slide are formed as slots whichin all of said slide''s positions for controlling the level of theplough in dependence of the pressure in the working chamber of thelifting cylinder communicate with said constriction means adjusted independence of the pressure in the working chamber and with the controlport in the slide face connected to the drain chamber, respectively. 6.An apparatus as claimed in claim 1, wherein said second slide isprovided with a control recess communicating with the control pressurechamber, and said first slide has a control slot which upon rotation ofsaid first slide to a range of positions, where the connection betweenthe control pressure chamber and the constriction means adjusted independence of the pressure in the working chamber of the liftingcylinder is cut off, communicates with the control port in the slideface connected to the drain chamber.